1. Field of the Invention
The present invention relates to a photoelectric conversion device and, more specifically, to a photoelectric conversion device comprising amorphous silicon photodiodes and suitable for application as an image sensor to a facsimile equipment or an image processing apparatus.
2. Description of the Prior Art
Zealous researches have been made in recent years to develop a photoelecric conversion device employing a photodiode array for use as an image reader for a facsimile equipment or an image processing apparatus. An amorphous silicon photodiode, for example, a photodiode using hydrogenated amorphous silicon (hereinafter referred to as "a-Si:H") is one of the photodiodes for such a device. Since a-Si:H photodiodes facilitate forming a large or long photodiode array, a-Si:H photodiodes are suitable for application to a contact type image sensor for isometric document reading. U.S. Pat. No. 4,419,696 and IEEE TRANSACTIONS ON COMPONENTS, HYBRIDS, AND MANUFACTURING TECHNOLOGY, Vol. CHMT-7, No. 4, pp. 423-428, Dec. 1984 disclose photoelectric conversion devices employing an array of a-Si:H photodiodes as shown in FIG. 1.
The a-Si:H photodiode shown in FIG. 1 is a Schottky barrier diode formed by superposing a metallic electrode 2 formed of Cr (chromium), an i- or p-type a-Si:H layer 3 and a transparent electrode 4 formed of ITO (indium tin oxide) in that order on an insulating substrate 1. In reading the image of a document by this photodiode, a power supply 5 maintains the transparent electrode 4 at a negative bias potential, and a driving circuit 6 maintains the metallic electrode 2 at a ground (or positive) bias potential. That is, in the conventional photoelectic conversion device using a-Si:H photodiodes, a bias voltage is applied across the metallic electrode 2 and the transparent electrode 4 so that the transparent electrode 4 is maintained at a negative bias potential relative to the metallic electrode 2. Accordingly, a Schottky barrier is formed in the interface between the a-Si:H layer and the transparent electrode 4, and the Schottky barrier functions as an electron blocking layer for making the interior of a light receiving part, namely, the a-Si:H layer, a depletion layer. This known photodiode advantageously requires a simple manufacturing process because only a single a-Si:H layer is necessary and any insulating film need not be formed between the a-Si:H layer 3 and the transparent electrode 4. However, the condition of the Schottky junction between the transparent electrode 4 and the a-Si:H layer 3 is greatly dependent on the composition and forming condition of ITO forming the transparent electrode 4, and the surface condition of the a-Si:H layer, and hence it is difficult to form a stable Schottky junction. Accordingly, it has been difficult to read the image of a document at a high S/N ratio with the photodiode by applying a bias voltage to the photodiode.